Field of the Invention
The present invention relates to an electrophotographic photosensitive member of a laminar structure, and more particularly, to such photosensitive member having at least a charge transport layer and a charge generation layer in succession in this order on a conductive substrate, and an imaging process using such photosensitive member.
Description of the Prior Art
There is already known an electrophotographic photosensitive member in which the photosensitive layer is functionally divided into a charge generation layer and a charge transport layer, and in which certain photoconductive disazo pigments are utilizing as the charge generation material.
In general, such photosensitive member is provided with a charge generation layer and a charge transport layer superposed in this order on a conductive substrate, in which said charge transport layer contains a strongly electron-donating charge transport material to facilitate the transportation of positive charge, and such photosensitive member is negatively charged in the imaging process.
This is due to a fact that negative charge transporting materials are generally unsatisfactory in performance and are frequently unacceptable for commercial applications due to strong carcinogenicity thereof.
However, negative corona discharge results in formation of significant ozone, giving rise to an additional cost for an ozone filter in the copying machine for removing such ozone. Also periodic maintenance becomes indispensable since such ozone filter deteriorates gradually in performance in the prolonged use.
Besides the negative corona discharge tends to result in an uneven discharge, for example due to smears on the discharge wires, thus leading to uneven image density. Also the generated ozone undesirably affects the service life of the organic photoconductor.
Furthermore, the negative corona discharge is associated with deterioration of the surface of photoconductor by the generated ozone and deposition of ionic substances, generated by the discharge, onto the photoconductor, whereby the surface potential thereof is entirely or locally reduced, thus eventually causing entire or local blurs or defects in the electrophotographically reproduced image.
On the other hand, positive corona discharge generates ozone in an amount of 1/5 to 1/10 in comparison with the case of negative corona discharge, is much less associated with uneven discharge caused, for example, by the smears of the discharge wires, and is more desirable for the service life of the photosensitive member. Because of such drawbacks in the negative charging, the development of a positively chargeable photosensitive member has been desired.
A positively chargeable laminar-structure photosensitive member can be constructed, for example, by superposing a positive charge transport layer and a charge generation layer in this order on a conductive substrate.
The charge generation layer is usually made as thin as 0.1 to 0.5.mu., since a larger thickness leads to various drawbacks such as an enhanced photomemory or an increase in the potential in the light portion after repeated use, due to the trapping of photo-generated carriers in such thicker charge generation layer.
Also the ratio of the charge generation material to the binder is usually selected in a range from 1/1 to 3/1, since a larger content of the binder will reduce the efficiency of carrier injection from the charge generation layer to the charge transport layer, thus leading to losses in sensitivity and memory characteristics.
However, a charge generation layer, composed of dispersion of fine particles, will show a poor mechanical strength if such layer is used as the outermost layer. In a copy process for example consisting of the steps of electrostatic charging, imagewise exposure, image development, transfer of toner image onto a transfer material such as paper or plastic film, separation of the transfer material from the photosensitive member, cleaning and charge elimination before or after the cleaning, the surface of a photosensitive member utilizing such outermost charge generation layer will be gradually abraded off for example in the steps of image development, image transfer and cleaning in which said photosensitive member is brought into contact with other mechanical parts. Consequently, in the prolonged use, the photosensitive member will develop significant surface damages and a significant change in the sensitivity, or in extreme cases, the charge generation layer will be abraded off so that the photosensitivity will be entirely lost.
On the other hand, an increase in the binder content in the charge generation layer, for increasing the physical strength of the surface of photosensitive member as a countermeasure against such drawbacks, will significantly disturb the carrier transportation in the charge generation layer, leading to a lowered sensitivity, an increased photo-memory and an increased light portion potential after prolonged use.
The conventional charge generation layer is mechanically not strong enough as the surface layer and is practically unsatisfactory for use in a copying machine or a printer because of poor abrasion resistance. The abrasion is caused by various reasons but mainly by surface cleaning, particularly cleaning with a blade.
The surface deterioration in the prolonged use is initially caused by scars on the photosensitive member, and this is followed by adhesion of toner particles and deposition of talc or other substances contained in the dust coming from the transfer sheet. These deposited substances may be decomposed for example by corona discharge, thus reducing the surface resistance, particularly when moisture is absorbed in the surface, and eventually resulting in so-called image flow. In extreme case a loss in the sensitivity may result from the gradual abrasion of the surface layer.